1. Field of Invention
The present invention relates to technical field of equipments for petroleum and gas drilling engineering, mining, architectural engineering, tunnel engineering, shield tunneling and other trenchless engineering, and more particularly to a roller bit for rock breaking by rotary cutting.
2. Description of Related Arts
Bit is a tool which directly contacts rocks and breaks the rocks by cutting, stamping and etc. The bit widely applied in modern drilling engineering mainly includes the roller bit comprising the single roller bit and the three roller bit, and the PDC bit (Polycrystalline Diamond Compact Bit). Nowadays, both the roller bit and the PDC bit are highly developed, but there are still some insurmountable problems. A first problem thereof is that reaming may cause premature failure of the roller bit, especially the PDC bit, of a sealed bearing. A second problem thereof is that the bit causes small motion amplitude of the rocks in the center of the borehole bottom, and has insufficient function, so that the rocks are hard to break thereby, which affects penetration rate of the drill. Furthermore, the roller bit and the PDC bit have small amount of teeth provided thereon and are easy wearing, which are also the main reason for the failure of the bit.
During the process of drilling, the three roller bit breaks the rocks by impacting and crushing, wherein the teeth on the roller has an effect of impacting and crushing on rocks at the bottom hole, so as to break the rocks thereof and form a borehole diameter. Since the rocks has a much greater compressive strength than the shear and tensile strength and the three roller bit breaks the rocks utilizing the impacting effect of the teeth on the rocks, energy efficiency thereof is not high. Especially in condition with high bit pressure such as deep drilling, under the effects of the high-density drilling fluid, chip hold down effect of rock debris at the bottom hole is obvious, so it is difficult for the teeth of the bit to penetrate the rocks to impact and break the rocks. Thus rock-breaking efficiency of the three roller bit is relatively low.
Bearing life is one of the main factors restricting the life of the three roller bit. Since the bearing bears a load caused by the bit pressure, the three roller bit breaks rocks by impacting, the bearing bears a large impacting load and amplitude of the load is high. In addition, the three roller bit has an effect of speeding up, cone/bit speed ratio (a ratio of a rotating speed of a roller to a rotating speed of a bit while drilling) of the three roller bit is over 1, i.e., when the bit drills, the rotating speed of the roller is greater than the rotating speed of the bit, so the rotating speed of the bearing is relatively high, which causes that the bearing of the three roller bit has a short service life.
Roller distribution of a double-cone bit is the same with thereof the three roller bit. Working faces (cambered surfaces for distribution of teeth) of all rollers are all provided in opposite, i.e., all provided inward. Besides the problems mentioned above, analyzing the rock breaking process and the movements of the double-cone bit, the time for the teeth on the roller to contact rocks at the bottom hole is very short, and the sliding distance of the teeth at the bottom hole is small, so efficiency of rock breaking in the center is low, which affects rate of penetration.
Unlike the cono-spheroid (approximately hemispherical) hemisphere shape of the double-cone bit and the three roller bit, the roller shape of the single roller bit is nearly spherical, and working faces thereof are fully distributed on the whole spherical surface while working faces of the double-cones and the three rollers are half-arc surfaces. The single roller bit breaks rocks by impacting, rolling and scraping of the teeth on the stratum. In particular, directions of scraping movement are different in various part of the roller surface, i.e., different rows of teeth cut the stratum by sliding in a long distance along tracks in different directions, in such a manner that reticular tracks at the bottom hole are formed. Characteristics of the single roller bit is low rotating speed of the roller, large size of bearing and longer service life than the three roller bit. However, the single roller bit still has unavoidable shortcomings as follows. The packed structure of the single roller bit determines that repeated breaking and chip removal difficulty frequently occur during the process of rock breaking. Wear resistance of the teeth is gravely insufficient, and once blunt or broken occurs on the teeth, the penetration rate decreases sharply.
The PDC bits break the rocks by scraping, are widely applied in engineering of drilling, geological and construction and are getting a larger and larger application ratio during the drilling process. Under an ideal working condition that the center line of the bit is overlapped with center line of the borehole, motion trails of each tooth is a relatively fixed concentric rings girdle. Due to the differences in rock breaking mechanism and structures thereof, the PDC bits are suitable for a high drilling speed and a soft to medium hard stratum. Shortcomings of the PDC bit mainly lies in the following aspects.
1. High Requirement for the Bottom Hole Condition
If the bottom hole has external matter, tooth breakup or friction heat occurs on the PDC bit, a raised temperature scorches and blackens the matrix, or even melts the brazing layer, so that a phenomenon of teeth loosing happens, which affects the penetration rate and accelerates the failure of the bit.
2. Serious Consequences of Failure of the PDC Teeth
Compared with the roller bit, the failure of individual teeth, such as teeth lost and teeth broken, significantly increases the load of the neighborhood PDC teeth, accelerates wearing rate thereof and exacerbates the failure of the bit.
3. Low Rock-Breaking Efficiency in a Center Portion
The wear rate of PDC teeth in different radial areas of the PDC bit is significantly different. The external shoulder portion of the PDC bit cuts a larger proportion of the rocks and wear rate thereof is higher. The PDC teeth in the cone portion have a lower efficiency for contacting and breaking the rocks and thus wear rate thereof is lower.